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United States Patent |
5,066,503
|
Ruozi
|
November 19, 1991
|
Method of pasteurizing or sterilizing foodstuffs utilizing microwaves
Abstract
Foodstuff packaged in a material transparent to microwaves is conveyed
continuously through an environment pressurized to 2.5 bar max so as to
prevent the containers or wrappers bursting or being forced out of shape;
in four steps, the temperature is raised quickly to a prescribed treatment
value with high frequency microwave radiation, stabilized by applying a
combination of lower power microwaves and hot air, held steady with hot
air only, and then lowered by cooling. In a split-level cylindrical oven,
the upper level consists in a pressure-and-temperature compensating
chamber (4), a high speed heat chamber (5) with closely spaced microwave
generators (24) ordered in longitudinal rows along its top and sides, the
waveguides (24', 25) of which are directed at the containers or wrappers
(C) of foodstuff from above and beneath as they are indexed through on
special pallets (S), and a stabilization chamber (5) in which lower power
microwaves combine with air drawn through heaters (20) and circulated by a
fan (21) located at the closed end of the oven; the same air is used to
heat a vertical chamber (6), through which the pallets descend on an
elevator (D) before passing into a cooling chamber (7) on the lower level
and proceeding toward the final outgoing compensating chamber (8). Once
out of the oven, the pallets (S) are raised by a further elevator (A) to
the upper level for emptying and reloading.
Inventors:
|
Ruozi; Giuseppe (Reggio Emilia, IT)
|
Assignee:
|
Officine Meccaniche Attrezzature per Ceramiche (IT)
|
Appl. No.:
|
611563 |
Filed:
|
November 13, 1990 |
Foreign Application Priority Data
| Jun 07, 1988[IT] | 40090 A/88 |
Current U.S. Class: |
426/234; 426/241; 426/521 |
Intern'l Class: |
A23L 003/00 |
Field of Search: |
426/234,243,521,241
219/10.55 M
99/451,DIG. 14
|
References Cited
U.S. Patent Documents
3889009 | Jun., 1975 | Lipoma | 426/243.
|
3961569 | Jun., 1976 | Kenyon et al. | 426/234.
|
Primary Examiner: Yeung; George
Attorney, Agent or Firm: Lowe, Price, LeBlanc and Becker
Parent Case Text
This application is a divisional application of application Ser. No.
07/360,798, filed June 2, 1989.
Claims
What is claimed:
1. A method of pasterizing or sterilizing packaged foodstuffs using
microwaves at 2.450 MHz frequency, comprising the steps of:
(a) batching or arranging the liquid or solid edible product in containers
or wrappers wholly transparent to microwave radiation;
(b) introducing the containers or wrappers into and conveying them
continuously through an environment that is pressurized to a level of up
to 2.5 bar max, dependent upon the properties of the edible product, in
order to prevent the containers or wrappers from bursting or becoming
distorted, and rendered multiresonant by a diffusion of microwaves from
above and below;
(c) raising the temperature of the product initially in said pressurized
environment;
(d) stabilizing the temperature reached by initial heating using circulated
heated gas in combination with microwave radiation in said pressurized
environment;
(e) holding the stabilized temperature using circulated heated gas in said
pressurized environment; and
(f) cooling the edible product following sterilization or pasteurization in
said pressurized environment.
2. A method as in claim 1, wherein step (c) effected internally of the
pressurized environment is one of heating the edible product by exposure
to microwave radiation from above and below for a period of approximately
4 to 10 minutes, dependent upon the properties of the foodstuff and the
pressure and temperature conditions existing within the environment, until
a predetermined temperature value for pasteurization or sterilization is
reached.
3. A method as in claim 1, wherein step (d) effected internally of the
pressurized environment is one of exposing the edible product
simultaneously to microwave radiation reduced intensity and to hot air
circulated at a temperature of 90.degree. to 125.degree. C. or more for a
period of approximately 2 to 4 minutes, dependent upon the properties of
the foodstuff and the pressure and temperature conditions existing in the
environment, so that a temperature internally of each container or wrapper
is stabilized at a predetermined value.
4. A method in claim 3 wherein step (c) effected internally of the
pressurized environment is further one of maintaining the edible product
at the stabilized predetermined temperature by continued exposure to hot
air circulated at 90.degree. C. to 125.degree. C. or more for a period of
approximately 1 to 10 minutes.
5. A method as in claim 1, wherein step (f) effected internally of the
pressurized environment is one of cooling the edible product by exposure
to circulating cold air to lower the temperature to approximately
50.degree. to 60.degree. C., according to the pressure existing in the
environment and to the properties of the foodstuff.
Description
BACKGROUND OF THE INVENTION
The invention disclosed relates to a method of pasteurizing and sterilizing
foodstuffs utilizing microwaves, and to an oven for implementing such a
process, that is, a new microwave process by which to reduce or eliminate
bacteria from foods for human and animal consumption, and thus prolong the
period for which they can be preserved; both the method and the oven used
for its implementation constitute the subject matter of the application.
In addition to the long-standing hot air, hot water and vapor methods of
pasteurization (85.degree. C.) and sterilization (121.degree. C.), and to
methods of deep freezing (-25.degree. C.), the prior art also embraces a
type of treatment utilizing microwaves generated at the conventional
frequency of 2.450 MHz.
Traditional hot air, hot water and vapor methods are beset by several
drawbacks, namely long exposure times, lack of continuity, a cooked effect
produced by the high temperatures adopted to speed up the process,
non-uniform temperatures, deterioration of or damage to wrappings, and
unsatisfactory results in the case of solid foods, due to the slow
transfer of heat by conduction.
There are also drawbacks with the deep-freezing of food-stuffs, most
notably the greater complexity and cost of equipment as compared with that
utilized for pasteurization and sterilization.
Microwave treatment of foodstuffs involves the application of heat by
inducing molecular vibration throughout the entire mass of the edible
substance, and thus brings a number of theoretical advantages:
pasteurization or sterilization can be achieved in a much shorter time
than is possible by applying heat directly, the organoleptic properties of
the food are improved, as also is its appearance and that of its wrapping;
labor costs are cut by virtue of the automation obtainable; the cooked
effect is reduced; sterilization of solids can be achieved without the
need to use regulating fluids, and pasteurization is made possible without
excessive damage occurring to wrappings as a result of internal pressure
generated by evaporation and heat. The only disadvantage is that the
wrappings utilized must be transparent, wholly or in part, to microwave
radiation.
Notwithstanding these apparent advantages, the attempts made thus far at
microwave pasteurization and sterilization on an industrial scale have not
been such as to develop a continuous pasteurization method whereby
packaged foodstuffs can be subjected to uniform and stable heat throughout
the entire area of the oven; neither has sterilization been successful,
inasmuch as the requisite temperature level tends to produce internal gas
pressures of an order that often cause the wrapping or container to
explode.
Accordingly, microwave methods do not constitute a viable proposition at
industrial level.
The prior art as summarized above thus stands in need of considerable
improvement, the objects being to eliminate the aforementioned drawbacks
besetting microwave pasteurization, and to enable the use of microwave
radiation in effecting sterilization on industrial scale.
It will be discerned from the foregoing that the requirement exists for a
new method of microwave treatment capable of being operated continuously,
of ensuring that each container or wrapper containing the foodstuff will
be invested with a constant, uniform heat, according to the temperature
selected, without bursting or becoming distorted (even at high
sterilization temperatures), and of turning out an end-product
substantially fresher than is obtainable conventionally, in less time and
with less waste.
SUMMARY OF THE INVENTION
The stated objects are achieved by adoption of a microwave pasteurization
or sterilization method using high frequency microwaves, say of 2.450 MHz;
the liquid or solid foodstuff for treatment, batched into containers or
wrappers completely transparent to microwaves, is fed into and conveyed
forward through and environment pressurized in such a way as to set up a
balancing pressure, maximum 2.5 bar for example, that will prevent the
container or wrapper from bursting or distorting as a result of the heat
generated by high frequency radiation.
The product is exposed to microwaves from above and below for an initial
high speed heat step, say, of 4-10 minutes duration, in which the
temperature rises to a prescribed level; this is followed by a second step
that serves to stabilize the initial temperature rise, and involves
further application of microwaves from above and below, in this instance
less intense and accompanied by hot air, for example between 90.degree.
and 125.degree. C. and circulated for a period of approximately 2-4
minutes, to the end of obtaining an identical temperature internally of
each wrapper or container. A third step involves maintaining the
temperature uniform for between 1 and 10 minutes by continued circulation
of hot air. In the fourth and final cooling step, the product is invested
with cold air in such a way as to bring the temperature down to
approximately half the level of the former steps, or at all events to a
level that will ensure there is no danger of the containers or wrappers
bursting or distorting.
Pressure levels are compensated during passage of the product from
atmospheric to internal pressurized conditions, and viceversa, as also are
temperature levels, particularly at the entry stage.
The oven used to implement the method disclosed consists in a longitudinal
cylindrical tunnel type structure divided horizontally into two distinct
levels: an upper level, through which the containers or wrappers filled
with the foodstuff travel during the initial steps, arranged in
rank-and-file on pallets and propelled step by step along tracks by
horizontal rods, and a lower level of smaller volume along which the
return passage is effected in the same fashion.
Single pallets bearing the product are propelled along the upper level by
pairs of reciprocating rods: a first pair of rods passing through a hinged
airtight flap, which push the pallet into an initial pressure compensating
chamber prior to its entering the oven; a second pair of rods effecting
passage from the chamber through a further hinged airtight flap into the
oven proper; and a third pair of rods by which the column of pallets is
propelled through to the end of the heat generation and temperature
stabilization steps of the process.
Maintained substantially at the temperature and pressure levels existing in
the oven proper, and capable of accomodating the contents of one pallet,
the compensating chamber is equipped with valves that admit and discharge
pressurized air, and a pair of tracks across which the single pallets pass
in a longitudinal direction from the tracks at the entry side onto those
at the exit side. The compensating chamber leads into a resonant heat
chamber served by medium-low power microwave generators (e.g. adjustable
between 0 and 1200 W in the initial high speed heat section) arranged in
closely spaced rows along the top and side of the cylinder; in a preferred
embodiment, the three rows uppermost will be spaced apart approximately 40
cm apart lengthwise, one at center and the other two at 45.degree. on
either side, with two additional rows set at 30.degree. farther around the
circumference of the cylindrical structure (which will be some 2 meters in
diameter).
The generators of the top three rows have hollow wave-guides of rectangular
section that project down radially toward the product, whilst the
waveguides of the side rows project radially at first and are then
diverted horizontally, converging ultimately at center. Beyond the high
speed heat chamber in the stabilization chamber, use is made of lower
power microwave generators (max 1000 W as against 1200 W), set farther
apart and in alternation, combined with hot air drawn in from the top part
of the tunnel, above the level of the conveyed product, by a fan unit
located at the end of the cylindrical structure (which will be some 12
meters in length); the air is directed over a baffle through a battery of
electric heaters positioned in the top of the tunnel, taken in by the fan
and blown at the oncoming pallets, passing through a descending elevator
that occupies a vertical chamber located between the stabilization chamber
and the fan end of the oven. The elevator receives one pallet-load of the
product with each indexed step of the propulsion system, transferred into
position by reciprocating rods with pivoted catches, and at the same
moment releases the pallet occupying its bottom tier, which is transferred
through a hinged airtight flap onto the tracks of the lower return level
of the oven.
Passage through the flap is produced by further reciprocating longitudinal
propulsion rods with pivoted catches that move longitudinally across the
bottom of the vertical chamber above a longitudinal baffle which, together
with the flap, separates the vertical hot air chamber from the cooling
chamber; the rods engage each of the descending pallets in turn and propel
them nose to tail back through the cooling chamber toward the exit of the
oven.
Blown horizontally through the descent elevator, hot air is gathered
between the separating baffle at bottom and a further baffle above, and
directed back into the stabilization zone, passing beneath the oncoming
pallets then rising around and between them to return toward the heaters.
The bottom part of the oven serves to effect the cooling process. Pallets
carrying the containers or wrappers, transferred from the bottom tier of
the descent elevator and propelled along the track, are invested with cold
air from transversely positioned fan units located at bottom; the
temperature of the product is brought down to 50.degree.-60.degree. C. or
thereabouts by the cold air stream, the cold draft being favored by
transverse ridges formed in the pallet surface.
On reaching the end of the return stretch, the leading pallet is engaged by
a pair of reciprocating rods with pivoted catches and transferred into the
outgoing compensating chamber, which is situated beneath the incoming
chamber and provided similarly with inlet and outlet air valves, tracks,
and hinged airtight flaps; the pressure level in this chamber is identical
to that existing in the oven, and the temperature identical to that of the
cooling stage.
The single pallets are transferred by further push rods out of the
compensating chamber and onto a pair of longitudinal horizontal belt loops
which index them toward the bottom tier of an ascending elevator (lying in
the same plane as the bottom tier of the descending elevator), then raised
ultimately to a position that coincides with the upper level on which
entry into the oven occurs, but is set back from the actual entry point.
The pallet is now indexed forward by a pair of push rods onto a pair of
tracks forming part of the upper level, moving through a distance
corresponding to the depth of the ranks of containers or wrappers in which
the foodstuff is batched, whereupon the single containers or wrappers
themselves are picked up by suckers mounted to a swing arm, and set down
in single file on a transverse runout line.
As the rods continue indexing forward, the empty pallet is filled again,
rank by rank, from an infeed line disposed parallel to the runout and
served by an identical swing arm with suckers, refilled, the pallet is
propelled forward by the same rods into the upper compensating chamber.
Advantages of the invention are: uniform heat treatment through the
different areas of the oven, and from container to container;
stabilization to a prescribed temperature effected in short intervals of
time and space; instant adaptability to different production cycles given
by extensive adjustment in power output of the individual microwave
generators; constantly repeated pasteurization, and no rejects; ease and
speed of sterilization on industrial scale, with no deterioration either
of the packaging or of the end-product; low running costs.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in detail, by way of example, with the
aid of the accompanying drawings, in which:
FIGS. 1, 2 & 3 are longitudinal vertical sections of the oven illustrating
the zones in which the various steps of the method are implemented,
respectively: infeed/runout and pressure/temperature-compensation (FIG.
1); high speed heat, temperature stabilization and cooling (FIG. 2);
again, heat and stabilization, and subsequent holding of the stable
temperature (FIG. 3);
FIG. 4 is the vertical cross section through IV--IV in FIG. 2, illustrating
the high speed heat zone;
FIG. 5 is a horizontal longitudinal section through V--V in FIG. 1,
illustrating the system of push rods operating on the upper level of the
oven, externally at the runout and infeed station, and internally of
pressure-and-temperature compensating chamber;
FIG. 6 is a longitudinal horizontal section through VI--VI in FIG. 1,
illustrating the passage through the outgoing pressure-and-temperature
compensating chamber and the lower level of the runout station.
DESCRIPTION OF THE PREFERRED EMBODIDMENTS
In the drawings, 1 denotes the protective outer casing of the entire oven
installation, for example, embodied in stainless steel; this encompasses a
cylindrical metallic skeleton structure 2, and attached thereto, an
internal sheet metal lining 2' shaped to form a cylindrical enclosure and
lagged externally with heat insulating material 3. 4 denotes the incoming
pressure-and-temperature compensating chamber, the capacity of which is
equal to the contents of one pallet-load of the edible product. 5 denotes
the initial high speed heat chamber, and 5' the chamber in which the
temperature reached in the first chamber is stabilized, and internally of
which pasteurization or sterilization of the end-product effectively takes
place. 6 denotes the chamber in which stable pasteurizing or sterilizing
temperature is held steady for a given prescribed duration. 7 denotes the
chamber in which the product is cooled, and 8 the outgoing compensating
chamber, likewise able to accommodate one pallet.
Single containers C of the edible product to be pasteurized or sterilized
are conveyed toward the oven along an infeed line AL and transferred into
special pallets S, each one of which exhibits a bearing surface fashioned
in polycarbonate material transparent to microwaves, a teflon frame
(likewise transparent), and a right longitudinal section of ribbed
profile; the transfer is effected by a swing arm 9 with suckers.
The pallet S is made to slide along a pair of longitudinal tracks 10, also
in teflon, by means which comprise a pair of longitudinally disposed
pneumatic cylinders 11 located one at either side of the tracks and
associated in turn with a pivoted catch in such a way as to create a pair
of actuators 12 that index the pallet S through a longitudinal path into
the compensating chamber 4.
PI denotes a hinged flap affording entry to the compensating chamber 4 and
incorporating sections of track that align with a further pair of
longitudinal teflon tracks 13 inside the chamber 4 itself, thus providing
a continuation of the external tracks 10.
PU denotes a hinged flap at the exit end of the compensated chamber 4,
embodied in similar fashion to the entry flap PI and aligned with a pair
of longitudinal teflon tracks 14 running the entire length of the two
chambers denoted 5 and 5'.
15 denotes a second pair of longitudinal push rods, operated by belt-driven
propulsion units 16 in this instance, that serve to transfer the pallets S
from the compensating chamber 4 to the heat chamber 5 and stabilization
chamber 5'; once inside the heat chamber 5, the pallets S are propelled
through the oven nose-to-tail by a third pair of longitudinal push rods
17, also belt-driven, that impinge on the trailing edge of the hindmost
pallet.
D denotes an intermittently operated descent elevator occupying the
vertical chamber 6, which consists in tiered pairs of bearers associated
with respective mutually opposed chain loops and receives the pallets S
from the longitudinal tracks 14 of the upper level and transfers them to a
further pair of longitudinal tracks 18, also teflon, running through the
cooling chamber 7 on the lower level.
19 denotes a pair of actuators consisting in pivoted catches operated by a
piston and a pair of rack mechanisms, that serve to separate the leading
pallet S of the column and position it on the vacant pair of bearers
offered by the elevator D.
20 denotes a set of electrical resistance heater elements occupying a
position at the top of the oven in a restricted terminal part of the
stabilization chamber 5'; air drawn by a fan unit 21 through these heater
elements is channeled through the center rear opening 22 of a longitudinal
baffle 23 that extends rearward through the restricted zone and down
toward the fan unit 21. Blown by the fan into the vertical chamber 6, the
air is made to circulate beneath the pallets S occupying the temperature
stabilization chamber 5'. The option exists of utilizing a heat exchanger
in place of the electrical resistances. 24 denotes one of a plurality of
magnetron type microwave generators arranged in longitudinal rows along
the top half of the cylindrical enclosure and passing through the sheet
lining 2' and insulation 3 to the interior; the generators 24 are radially
disposed and distributed along the entire length of the heat and
stabilization chambers 5 and 5', the three rows uppermost (see FIG. 4)
being associated with waveguides 24' that project inward radially toward
the containers C proceeding along the upper level. The waveguides 25 are
equipped with radial perforations and project inward radially for a brief
distance before extending into horizontal stretches that continue inward
beneath the pallets S carrying the containers C and converge at center.
26 denotes a longitudinal baffle at the bottom of the oven, separating the
vertical chamber 6 from the cooling chamber 7; this baffle 26 is
effectively a continuation of the downwardly directed part of the top
baffle 23, and is broken by a further hinged flap 27 through which the
pallets S pass into the cooling chamber. 28 denotes the cap enclosing the
end of the cylindrical structure 2. 29 denotes a pair of longitudinal rods
occupying the cooling chamber 7, interconnected at their two ends and
operated by a further pair of belt-driven propulsion units 16, the rear
ends of which carry a pair of pivoted catches 30 designed to engage the
pallet S occupying the bottom tier of the descent elevator D and transfer
it through the hinged flap 27 and onto the tracks 18 of the lower level.
31 denotes a plurality of transversely disposed cooling fan units forming
part of a bank of fin-fan heat exchangers.
32 denotes a pair of pivoted catches associated with a pair of
reciprocating rods, that serve to transfer the pallets S from the cooling
chamber 7 into the outgoing compensating chamber 8. PI' and PU' denote the
respective hinged entry and exit flaps affording passage to the pallets S
into and from this chamber 8, which are identical in all respects to those
PI and PU of the chamber 4 above; likewise, 33 denotes a pair of
longitudinal teflon tracks in the outgoing chamber 8 that are identical to
the tracks of the incoming chamber 4.
34 denotes a pair of catches operated by a power cylinder 35 (FIG. 6),
serving to transfer the single pallets S from the outgoing compensating
chamber 8 onto a pair of longitudinal belt loops 36.
A denotes an ascending elevator, identical in basic embodiment to the
descent elevator D, which receives the pallets S with their load of cooled
containers C from the belts 36 and transfers them back to the upper level,
where they are returned to the pair of tracks 10 first mentioned by a pair
of push rods 37 identical to the pair denoted 15.
38 denotes a swing arm with suckers identical to the arm 9 first mentioned,
by which the containers C filled with the now pasteurized or sterilized
edible product are transferred rank by rank onto a runout conveyor EV.
39 denotes a longitudinal and substantially diametral horizontal cavity by
which the upper and lower chambers 5 and 7 are separated thermally to
avoid the formation of hot spots. 40 and 41 denote two longitudinal
horizontal shelves, the first fashioned in stainless steel and
constituting the bottom wall of the upper multiresonant half of the
cylindrical enclosure, the second situated beneath the first 40, providing
the top wall of the cooling chamber 7; 42 are slabs of insulating material
identical to that denoted 3.
Operation of the oven will now be described. Containers or wrappers C of
the edible product to be pasteurized or sterilized approach in single file
on the infeed conveyor AL and are transferred rank by rank onto a pallet S
prior to entering the pressure-and-temperature compensating chamber 4,
which will be charged with air at the same pressure as that inside the
oven proper (max 2.5 bar for example), to the end of balancing the
internal pressure generated in the containers or wrappers (fashioned, say,
from a film material transparent to microwaves) by exposure to microwave
radiation as they pass along the chambers 5 and 5' in which pasteurization
or sterilization occurs; temperatures of the order of 100.degree. C. and
more are generated in the initial heat chamber 5, e.g.
120.degree.-140.degree. C. in the case of sterilization, and then
stabilized and held steady through the following chamber 5' with the aid
of hot air.
Once the pallet of containers C has covered a given distance, say 10
meters, taking between 4 and 10 minutes, enveloped through the final
stretch by hot air circulating at between 90.degree. and 125.degree. C.,
the prescribed stable treatment temperature will have been reached; the
pallet S is now transferred onto the descent elevator D, and after a
further period of between 1 and 10 minutes moving down through the
vertical chamber 6, still enveloped by hot air, passes into the low level
cooling chamber 7.
The cooling chamber is charged with cold air at a pressure identical to
that of the top chambers, which brings the temperature down to some
50.degree.-60.degree. C.; at this juncture, any danger of the container or
wrapper bursting or being forced out of shape can be discarded, and the
pasteurized or sterilized end product can be returned to atmospheric
conditions.
The pallet S is taken out through the appropriately conditioned outgoing
compensating chamber 8 and back to the top level, where the containers are
set down in single file on the runout line EV.
Uniformity of the heat treatment is ensured according to the invention by
the positioning of the waveguides, encircling the product as it proceeds
through the top chambers 5--5', and by the facility of regulating power
output of the single microwave generators; indeed, the generators will be
computer controlled in a preferred embodiment, so as to permit of
monitoring their output individually and effecting the appropriate
adjustments.
Similarly, the temperature of the hot and cold air supplies will be
computer controlled.
The embodiment described and illustrated is by no means limitative. For
example, the bottom sets of waveguides 25 need not necessarily converge at
the center of the oven, but might be staggered on either side of center.
Similarly, the maximum pressure of 2.5 bar is intended as a safe value to
balance the pressure generated internally of the container or wrapper, at
all events, in the case of general solid foods for human consumption;
nonetheless, this value might be varied to suit the properties of
different products for pasteurization or sterilization, to the same end of
ensuring that the containers or wrappers do not burst, collapse or
otherwise deteriorate.
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